Angular rate of rotation about a given coordinate axis may be measured by moving (e.g., vibrating) an accelerometer along an axis normal to the accelerometer's sensitive axis and normal to the rate axis about which rotation is to be measured. For example, consider a set of X, Y, Z coordinate axes fixed in a body whose rotation rate is to be measured, and an accelerometer also fixed in the body with its sensitive axis aligned along the Z axis. If the angular rotation vector of the body includes a component along the X axis, then periodic motion of the accelerometer along the Y axis will result in a periodic Coriolis acceleration acting in the Z direction that will be sensed by the accelerometer. The magnitude of the Coriolis acceleration is proportional to the rotation rate about the X axis. As a result, the output of the accelerometer includes a DC or slowly changing component that represents the linear acceleration of the body along the Z axis and a periodic component that represents the rotation of the body about the X axis. The accelerometer output can be processed, along with the outputs of accelerometers that have their sensitive axes in the X and Y directions and that are moved along the Z and X axes, respectively, to yield linear acceleration and angular rate about the X, Y and Z axes. Such signal processing is described in U.S. Pat. No. 4,445,376 and in U.S. Pat. No. 4,590,801 entitled "Apparatus for Measuring Inertial Specific Force and Angular Rate of a Moving Body".
As described in the latter patent, one preferred embodiment of a rotation rate sensor comprises, for each axis, two accelerometers oriented with their sensitive axes parallel or antiparallel to one another and means for vibrating the accelerometers along an axis normal to their sensitive axes. A suitable method for vibrating such accelerometer pairs is described in U.S. Pat. No. 4,510,802. In the system described in that patent, a parallelogram structure is used to vibrate the accelerometers along a common vibration axis. In such an arrangement, it may be demonstrated that angular misalignment of the two accelerometers with respect to the desired sensitive axis interacts with any phase shift that may exist between the vibration drive signal and the actual vibratory motion to produce a rate bias given by: EQU .OMEGA..sub.b =(.omega./4) (.alpha..sub.1 +.alpha..sub.2)sin .psi.(1)
where .omega. is the vibration frequency, .alpha..sub.1 and .alpha..sub.2 are the misalignments of the first and second accelerometers respectively and .PHI. is the phase shift. The referenced application describes a process, using the characteristics of a parallelogram linkage, to set (.alpha..sub.1 +.alpha..sub.2) and .PHI. essentially to zero. However to achieve a bias stability of, for example, 0.02 degrees/hour, it is still necessary that the phase shift and misalignment angles be stable to the order of 15 microradians each. In a gas damped accelerometer, where phase shift at the vibration frequency may be a few degrees, and where stability better than a few hundred microradians cannot be guaranteed, alignment stability becomes extremely critical, and an improved technique is desirable for minimizing the rate bias set forth in Equation (1) above.